This invention relates generally to electrostatic latent image development systems that operate using liquid developing material, and, more particularly, relates to a system for electrostatic development of a latent image, wherein the latent image is developed with use of a toner cake layer having a high solids content.
A typical electrostatographic printing process includes a development step whereby developing material including toner or marking particles is physically transported into the vicinity of a latent image bearing imaging member, with the toner or marking particles being caused to migrate via electrical attraction to the image areas of the latent image so as to selectively adhere to the imaging member in an image-wise configuration. Various methods of developing a latent image have been described in the art of electrophotographic printing and copying systems. Of particular interest with respect to contact electrostatic printing systems is the concept of forming a thin layer of liquid developing material on a first surface of a first member, wherein the layer has a high concentration of charged marking particles. The layer on the first member is brought into contact with an electrostatic latent image on a second surface of a second member, wherein development of the latent image occurs upon separation of the first and second surfaces, as a function of the electric field strength generated by the latent image. In this process, toner particle migration or electrophoresis is replaced by direct surface-to-surface transfer of a toner layer induced by image-wise fields.
Exemplary patents which may describe certain aspects of electrostatic and electrostatographic printing, as well as specific apparatus therefor, may be found in U.S. Pat. Nos. 4,504,138; 5,436,706; 5,596,396; 5,610,694; and 5,619,313, the disclosures of which are incorporated herein by reference.
It is desirable that the aforementioned layer of liquid developing material be provided in a very thin and very uniform layer that exhibits a high proportion of solids, that is, having a high solids content. Even more desirable is such a layer exhibiting the following advantageous characteristics: a selectable, uniform thickness, preferably in the range of 3-10 microns; a high solids content, preferably in the range of 15 to 35 percent solids; and an uniformly metered mass per unit area on the order of 0.1 mg per cm2.
The intuitive and conventional approach is to attempt the formation of such a layer by direct application of liquid developing material having a high solids content. However, due to the very complicated rheological behavior of a liquid developing material having the requisite high solids content, such direct application of a supply of such liquid developing material to a receiving member typically does not achieve a layer having the aforementioned desirable characteristics. For example, the resulting layer has been found to exhibit a variable thickness and a non-uniform mass per unit area, which renders the layer generally unsuitable for most electrostatic printing applications.
In accordance with one aspect of the present invention, there is provided an imaging system for effecting electrostatic printing of an image, wherein the imaging system includes at least one electrostatic printing engine operable in a novel fashion, wherein the electrostatic printing engine images and develops an electrostatic latent image representative of the image, and subsequently transfers the developed image to the copy substrate.
In accordance with another aspect of the present invention, a toner cake formation apparatus may be constructed and operated in accordance with the electrostatic printing process to which the present invention is directed, wherein a thin, uniform toner cake layer of high solids content is formed in a process nip between first and second movable members. The toner cake layer is generally characterized as having a high solids content (e.g., approximately 10-50 percent solids, and preferably in the range of approximately 15 to 35 percent solids, or greater), and exhibits the additional advantageous characteristics of a uniform thickness, in the range of 1-15 microns, and an uniformly metered mass per unit area in the range of 0.03-0.2 mg per cm2.
In accordance with another aspect of the present invention, an imaging system for effecting electrostatic printing of an output image may be constructed, wherein a first movable member is provided in the form of an imaging member having a latent electrostatic image on an image bearing surface, and the second movable member is provided in the form of a developed image receiving member. A toner cake layer of high solids content is formed in a process nip between the first and second movable members. A developed image is created as the toner cake layer exits the process nip, wherein portions of the toner cake layer separate in correspondence with the image and non-image regions of the latent image.
A preferred embodiment of the imaging system includes a supply of low solids content liquid developing material from which a low solids content liquid developing material applicator establishes a relatively uniform and constant aggregation of low solids content liquid developing material at the entrance of the process nip. The low solids content liquid developing material is a mixture of marking particles, such as toner particles, dispersed in a fluid carrier medium. This aggregation of low solids content liquid developing material is subject to compression in the process nip, such that the concentration of marking particles is increased in the process nip, and the concentration of carrier liquid is decreased in the process nip, thus causing formation of the desired toner cake layer.
In another aspect of the invention, a pre-development zone is established at the entrance of the process nip, wherein a controllable proportion of toner particles are believed to be preferentially capable of sustaining compression at the nip entrance so as to pass into the process nip. In contrast, a controllable proportion of the carrier fluid is believed to be preferentially restrained from entering the process nip. The increase in concentration of toner particles in the process nip thus yields a toner cake layer that is continuously formed therein.
In another aspect of the invention, the formation of the toner cake layer is accompanied by concurrent or near-concurrent development of the electrostatic latent image in a development zone situated in the process nip. The onset of formation of the toner cake layer is believed to occur during the forced migration of toner particles into the process nip. Complete formation of the toner cake layer is believed to occur concurrently or prior to the development of the latent image within the process nip, such that the developed image is completed upon separation of the toner cake layer into image and non-image portions at the process nip exit.
In accordance with another aspect of the present invention, an embodiment of a novel electrostatic printing engine may be constructed for imaging and development of a latent image, wherein the electrostatic printing engine includes an imaging member which is rotated so as to transport the surface thereof in a process direction for implementing steps for charging and formation of an electrostatic image corresponding to the desired latent image. A second movable member, in the form of a developed image receiving member, is provided in combination with an applicator of low solids content liquid developing material. The applicator establishes an aggregation of low solids content liquid developing material at the entrance of a process nip between the first and second movable members. Preferably, the aggregation is generally made up of toner particles immersed in a liquid carrier material and also typically including a charge director for providing a mechanism for producing an electrochemical reaction in the liquid developing material composition which generates the desired electrical charge on the toner particles. Movement of the imaging member and the developed image receiving member causes the toner cake layer to be formed in the process nip. As portions of the toner cake layer, which are subject to the electrostatic forces from the latent image, exit the process nip, a developed image, made up of selectively separated portions of the toner cake layer, is provided. Transfer of the developed image may then be accomplished.
The developed image may be provided on the imaging member, or, in a preferred embodiment, the developed image may be provided on the developed image receiving member. Accordingly, in the latter apparatus, a transfer station employing for high-temperature and pressure transfer and/or transfixing may be advantageously employed for carrying out the image transfer step from the developed image receiving member.
Accordingly, a preferred embodiment of an electrostatic printing engine may be constructed to include a movable photosensitive imaging member for receiving an electrostatic latent image. The imaging member includes a photosensitive surface capable of supporting a latent image, from which portions of the aforementioned toner cake layer are separated for subsequent transfer to a copy substrate. An imagewise exposure device is provided for generating the electrostatic latent image on the imaging member, wherein the electrostatic latent image includes image areas defined by a first charge voltage and non-image areas defined by a second charge voltage distinguishable from the first charge voltage. The apparatus is operated for forming the toner cake layer in the process nip between the surface of the imaging member and an adjacent receiving surface on a image receiving member. In response to the electrostatic latent image, developed non-image areas corresponding to the electrostatic latent image are provided on the imaging member, and developed image areas are provided on the receiving surface. Continued movement of the imaging member and the image receiving member causes separation of the toner cake layer in an image-wise manner. The developed image areas are then available for transfer to a copy substrate, and non-image (background) areas are removed from the imaging member.
In another aspect of the present invention, imagewise electric fields across the layer of toner cake are generated in the process nip. The process nip is defined by a nip entrance and a nip exit, wherein the process nip and the nip entrance are operative to apply compressive stress forces on the quantity of low solids content liquid developing material present therein, and the nip exit is operative to apply tensile stress forces to the toner cake layer, causing imagewise separation of the layer of toner cake in a pattern corresponding to the electrostatic latent image. The layer of toner cake is defined by a yield stress threshold in a range sufficient to allow the layer of toner cake to behave substantially as a solid at a development zone located between the nip entrance and the nip exit, while allowing the layer of toner cake along the boundary of the latent image and the image background to behave substantially as a liquid at the nip exit.
The toner cake layer is exposed to at least two stresses: a compressive stress in the process nip as well as at the entrance thereof; and a tensile stress at the nip exit as the developed image is separated into image areas on one surface and background areas on the other surface. In order to optimize the resultant image quality, it is desirable that the toner cake layer have sufficient yield stress to allow the toner particles therein to maintain their integrity while being exposed to these particular stress forces. Thus, pre-selecting materials having a particular yield stress and selectively controlling the compression forces applied to the aggregation of low solids content liquid developing material can assist in providing a self-sustaining process for formation of a toner cake layer having advantageous characteristics such as controlled thickness and density. These characteristics can be particularly useful in defining operational parameters for optimization of the electrostatic printing process.
Additionally, the electrostatic printing process of the present invention includes limited relative movement between toner particles during and after latent image development, wherein the high solids content of the toner cake layer prevents toner particles from moving relative to each other.